1. Field of the Invention
The present invention generally relates to heat recovery for the purpose of electrical or mechanical power generation. Specifically, the present invention is directed to various systems and methods for the conversion of heat of any quality into mechanical or electrical power.
2. Description of the Related Art
In general, there is a constant drive to increase the operating efficiency of heat and power recovery systems. By increasing the efficiency of such systems, capital costs may be reduced, more power may be generated and there may be a reduction of possible adverse impacts on the environment, e.g., a reduction in the amount of waste heat that must ultimately be absorbed by the environment. In other industrial processes, an excess amount of heat may be generated as a byproduct of the process. In many cases, such waste heat is normally absorbed by the environment through the use of waste heat rejection devices such as cooling towers.
There are several systems employed in various industries to produce useful work from a heat source. Such systems may including the following:
Heat Recovery Steam Generators (HRSG)—Typically, waste heat from gas turbines or other, similar, high quality heat sources is recovered using steam at multiple temperatures and pressures. Multiple operating levels are required because the temperature-enthalpy profile is not linear. That is, such prior art systems involve isothermal (constant temperature) boiling as the working fluid, i.e. water, is converted from a liquid to a vapor state. Various embodiments of the present invention eliminate the need for multiple levels and simplify the process while having the capability to recover more heat and to economically recover heat from a much lower quality heat source.
Rankine Cycle—The classic Rankine cycle is utilized in conjunction with HRSGs to produce power. This process is complex and requires either multiple steam turbines or a multistage steam turbine, feed water heaters, steam drums, pumps, etc. The methods and systems of the present invention are significantly less complex while being more effective than systems employing the Rankine cycle.
Organic Rankine Cycle—Similar to the classic Rankine cycle, an Organic Rankine cycle utilizes a low temperature working fluid such as isoButane or isoPentane in place of steam in the classic cycle. The system remains complex and is highly inefficient at low operating temperature differences.
Kalina Cycle—Dr. Kalina's cycle is a next generation enhancement to the Rankine cycle utilizing a binary fluid mixture, typically water and ammonia. Water and ammonia are utilized at different concentrations in various portions of the process to extend the temperature range potential of the cycle and to allow higher efficiencies than are possible in the Rankine cycle. The methods and systems of the present invention simplify the process while having the capability to recover more heat and to recover heat from a low quality heat source.
The system depicted in FIG. 5 is an example of a prior art system for heat recovery. The system comprises two heat recovery heat exchangers 120 and 121, two turbines (expanders) 122 and 124, and a reheater heat exchanger 123. The prior art system may or may not have a separate gas cooler 125 and condenser 126. The subcritical working fluid 102 enter the first heat recovery heat exchanger 120 at approximately the condensing temperature from a condenser 126. The liquid 102 is heated via heat transfer with the discharged hot fluid 114 from the reheater heat exchanger 123 and is discharged as either a wet or dry vapor 103 after boiling either partially or completely in heat recovery heat exchanger 120. The working fluid 103 is further heated in the second heat recovery heat exchanger 121 to a dry vapor 104 via heat transfer with the hot heat source 112 and is supplied to the inlet of the first turbine 122. In at least some cases, the vapor 104 is at a temperature near or slightly above its critical temperature but well below its critical pressure. The hot vapor 104 is expanded in turbine 122 and exits as a hot vapor 105. The hot vapor 105 is introduced into a reheater heat exchanger 123 where is heated (reheated) by the hot heating fluid 113 discharged from the second heat recovery heat exchanger 121 via heat transfer. The reheated working fluid 106 is then supplied to the inlet of the second turbine 124 wherein it is expanded and discharged as a hot, typically dry and highly superheated, vapor 107. The discharged vapor 107 from the second turbine 124 may or may not be cooled in a gas cooler 125 before being condensed in a condenser heat exchanger 126.
In the prior art system of FIG. 5, the subcritical working fluid 102 enter the first heat recovery heat exchanger 120 at approximately the condensing temperature from a condenser 126. Said liquid 102 is heated via heat transfer with the discharged hot fluid 114 from the reheater heat exchanger 123 and is discharged as either a wet or dry vapor 103 after boiling either partially or completely in heat recovery heat exchanger 120. Said working fluid 103 is further heated in the second heat recovery heat exchanger 121 to a dry vapor 104 via heat transfer with the hot heat source 112 and is supplied to the inlet of the first turbine 122. In the most preferred embodiment the vapor 104 is at a temperature near or slightly above its critical temperature but well below its critical pressure. The hot vapor 104 is expanded in turbine 122 and exits as a hot vapor 105. Such hot vapor 105 is introduced into a reheater heat exchanger 123 where is heated (reheated) by the hot heating fluid 113 discharged from the second heat recovery heat exchanger 121 via heat transfer. The reheated working fluid 106 is then supplied to the inlet of the second turbine 124 wherein it is expanded and discharged as a hot, typically dry and highly superheated, vapor 107. The discharged vapor 107 from the second turbine 124 may or may not be cooled in a gas cooler 125 before being condensed in a condenser heat exchanger 126.
The four largest weaknesses of the prior art system are a) the vapor 107 discharged from the second turbine 124 is significantly superheated and thereby the system of FIG. 5 fails to recover a portion of the valuable heat, b) the system utilizes a subcritical working fluid which limits the efficiency of the heat recovery in the heat recovery heat exchangers 120 and 121 due to the non-linearity of the temperature-enthalpy profile in said exchangers, c) the system generates unnecessary entropy further reducing its output in accordance with the Second Law of Thermodynamics, and d) the complexity of the system having multiple turbines and multiple heat recovery heat exchangers is reflected in an increased cost of the system for a given capacity. recovery heat exchanger(s) are usually the largest costs in a system of the type.
The following patents may be descriptive of various aspects of the prior art: U.S. Pat. No., 5,557,936 to Drnevich; U.S. Pat. No., 5,029,444 to Kalina; U.S. Pat. No. 5,440,882 to Kalina; U.S. Pat. No. 5,095,708 to Kalina; U.S. Pat. No. 5,572,871 to Kalina; Japanese Patent S53-132638A to Nakahara and Fujiwara; U.S. Pat. No. 6,195,997 to Lewis; U.S. Pat. No. 4,577,112 to Smith; U.S. Pat. No. 6,857,268 to Stinger and Mian; each of which are hereby incorporated by reference.
In general, what is desired are systems and methods for improving the efficiencies of various heat conversion and power generation systems and systems and methods for utilizing waste heat sources to improve operating efficiencies of various power and industrial systems. The present invention is directed to various systems and methods that may solve, or at least reduce, some or all of the aforementioned problems.